Exercise 5.20.  Draw the box-and-pointer representation and the 
memory-vector representation (as in figure 5.14) of the list structure 
produced by

(define x (cons 1 2))
(define y (list x x))

with the free pointer initially p1. What is the final value of free ? 
What pointers represent the values of x and y ?

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see 5.20.svg (or 5.20.png).